This invention relates generally to heat transfer printing sheets, and more particularly to a heat transfer printing sheet particularly suitable for producing an image on a heat transferable sheet or a sheet to be heat transfer printed by carrying out heat printing in accordance with image information by means of thermal heads, a laser beam, or the like.
Heretofore, a heat-sensitive color-producing paper has been primarily used to obtain an image in accordance with image information by means of thermal heads, a laser beam, or the like. In this heat sensitive color-producing paper, a colorless or pale-colored leuco dye (at room temperature) and a developer provided on a base paper are contacted by the application of heat to obtain a developed color image. Phenolic compounds, derivatives of zinc salicylate, rosins and the like are generally used as such a developer.
However, the heat sensitive color-producing paper as described above has a serious drawback in that its color disappears when the resulting developed color image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation.
On the other hand, a heat-sensitive transfer printing sheet wherein a heat-fusing wax layer having a pigment dispersed therein is provided on a base paper has been recently used. When this heat-sensitive transfer printing sheet is laminated with a paper to be heat transfer printed, and then heat printing is carried out from the back of the heat sensitive transfer printing sheet, the wax layer containing the pigment is transferred onto the heat transferable paper to produce an image. According to this printing process, an image having durability can be obtained, and a multi-color image can be obtained by using a heat-sensitive transfer printing paper each containing three primary color pigments in three different areas and printing it many times. However, it is impossible to obtain an image having an essentially continuous gradation as in a photograph.
In recent years, there has been a growing demand for a method and means for obtaining an image like a color photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand. One of such attempts provides a process wherein an image is projected onto a cathode-ray tube (CRT), and a photograph is taken with a silver salt film. However, when the silver salt film is an instant film, the running cost is high. When the silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing. An impact ribbon process and an ink jet process have been proposed as further processes. In the former, the quality of the image is inferior. In the latter, it is difficult to simply obtain an image like a photograph because an image processing is required.
In order to overcome such drawbacks, there has been proposed a process wherein a heat transfer printing sheet provided with a layer of sublimable disperse dyes having heat transferability is used in combination with a heat transferable sheet, and wherein the sublimable disperse dye is transferred onto the heat transferable sheet while it is controlled to form an image having a gradation as in a photograph. (Bulletin of Image Electron Society of Japan, Vol. 12, No.1 (1983)). According to this process, an image having continuous gradation can be obtained from a television signal by a simple treatment. Moreover, the apparatus used in this process is not complicated and therefore is attracting much attention.
One example of prior art technology close to this process is a process for dry transfer calico printing polyester fibers. In this dry transfer calico printing process, dyes such as sublimable disperse dyes are dispersed or dissolved in a solution of synthetic resin to form a coating composition, which is applied onto tissue paper or the like in the form of a pattern and dried to form a heat transfer printing sheet, which is laminated with polyester fibers constituting sheets to be heat transfer printed thereby to form a laminated structure, which is then heated to cause the disperse dye to be transferred onto the polyester fibers, whereby an image is obtained.
However, even if the heat transfer printing sheet heretofore used in the dry transfer calico printing process for the polyester fibers is used as it is and subjected to heat printing by means of thermal heads or the like, it is difficult to obtain a developed color image having a high density. The main reasons for this are that the heat sensitivity of the heat transfer printing sheet is not high and that the dyeability of the heat transferable sheet is low.
It has been found that, among these drawbacks, that attributable to the heat transferable sheet can be solved by a heat transferable sheet having a heat transferable layer which comprises mutually independent island-like portions formed from a synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C. and having a polar group and sea-like portions formed from a synthetic resin having a glass transition temperature of 40.degree. C. or above (Japanese Patent Application No. 135627/1983).
However, the drawback attributable to the heat transfer printing sheet has not yet been solved. In the prior calico printing process, the transfer and dying of the dye is accomplished by heating, for example, for about one minute at a temperature of 200.degree. C., whereas the heating pulse by means of thermal heads is short, i.e., of the order of several milliseconds at a temperature of about 400.degree. C.
In order to obtain a color photograph-like image by carrying out heat printing by means of thermal heads or the like, we have carried out studies to obtain a heat transfer sheet adapted for use in combination with a heat transferable sheet, particularly the heat transferable sheet of the foregoing Japanese Patent Application No. 135627/1983. As a result, we have made the following discoveries.
(i) In the heat transfer printing sheet heretofore generally used, the disperse dye is dispersed in the binder in the form of particles. In order to heat the dye molecules present in such a state to sublimate them, the dye molecules must be subjected to heat energy which breaks the interaction in the crystals and overcomes the interaction with the binder, thereby sublimating them to transfer to the heat transferable sheet. Accordingly, high energy is required.
(ii) When the dye is contained in a high proportion in the binder resin in order to obtain a developed color image having a high density, an image having a relatively high density can be obtained. However, its bond strength in the heat transfer printing layer of the heat transfer printing sheet becomes low. Accordingly, when the heat transfer printing sheet and the heat transferable sheet are peeled off after they are laminated and subjected to printing by means of thermal heads or the like, the dye tends to transfer to the heat transferable sheet with the resin.
(iii) The dye is expensive and the use of excessive dye is economically disadvantageous from the standpoint of office automation (OA) instruments and home uses.
On the other hand, if the dye can be retained in the binder in the form of molecules rather than particles, there will be no interaction in the crystals which occurs in the case where the dye is dispersed in the form of particles, and therefore an improvement in heat sensitivity can be expected. However, even if such a state is accomplished in the binder, a transfer paper having practicality cannot be obtained. This is because the molecular weight of the heat sublimable dye molecules is relatively small, i.e., of the order of from 150 to 500 and these molecules are liable to move in the binder.
Accordingly, when a binder having a low glass transition temperature (Tg) is used in a heat transfer printing layer, the dye agglomerates with elapse of time to be deposited. Eventually, the dye may be in the same state as the case where the dye is dispersed in the form of particles as described above. Alternatively, bleeding of the dye may occur at the surface of the heat transfer printing layer. Accordingly, the dye may be caused to adhere to portions other than the heated portions by the pressure between a thermal head and a platen during recording. Thus, staining may occur to significantly lower the quality of the image.
Further, even if the glass transition temperature (Tg) of the binder in the heat transfer printing layer is high, the dye molecules cannot be retained in the heat transfer printing layer unless the molecular weight of the binder is considerably high. Furthermore, even if the dye is dissolved in the form of molecules in a binder having a high glass transition temperature and a considerably high molecular weight, affinity between the dye molecules and the binder is required in order to achieve the state of storage stability.
In view of these findings, we have carried out further studies. As a result, we have now found that the prior art drawbacks can be solved at one stroke by using a specific compound as the binder in the heat transfer printing layer.